Carrier-signal frequency-detector system



Dec. 9,' 1941. H.,A. WHEELER I 'CARRIER-SIGNAL FREQUENCY-DETECTOR SYSTEM Filed Aug. l2, 1940 l l l l i I loyojndwo Y HAROLD A WHEELER BY Y Y ATTORNY Patented ec.. 9, i941 CARRIER- SIGNAL FREQUENCY-DETECTOR SYSTEM Harold A. 4 Wheeler, Great Neck, N. Y., assignor to Hazeltine Corporation,

a corporation of Dela- Applcation August 12; 1940, Serial N0. 352,211-

The present invention relates to an improved frequency detector system for carrier-signal receivers and, particularly, to such a system, the

output of which varies in 'substantially linear relation over a predetermined frequency range with the frequency of a carrier signal applied thereto. While the invention is of general application, it has particular utility detector system of a frequency-modulated carrier-signal'receiver. The invention, therefore;

q will be described in connection with a frequencyas the frequency modulated carrier-signal receiver wherein the inl vention is used to aid in obtaining freedom from distortion in the demodulation of the `carrier signal over 'a much greater range of carrierfrequency deviation than has heretofore been attainable.

Frequency modulation, as distinguished from amplitude modulation, of a carrier signal has certain advantages under some conditions of reception, among which is the reduction of objectionable interference whichmay be caused either by background noise or by an undesired station transmitting on the same or a nearby carrier frequency. For satisfactory reception of a frequency-modulated carrier signalLhowever, a detector having a substantially linear frequencyresponse characteristic over a predetermined deviation range of a receivedcarrier signal is essential. Furthermore, from the standpoint of sensitivity, it is desirable -that the characteristic curve of the detector should have a slope as great as possible over its linear range.

The ideal detector f or frequency modulation is one which not only faithfully reproduces the true. form ofthe frequency-modulation components but alsois unresponsive to amplitude fluctuations of the carrier signal. It has been proposed to reduce the effect of such amplitude uctuations of the carrier signal by the use of a balanced frequency detector comprisinga frequency-responsive network coupled to two rectil fier devices which convert the frequency-modulation' components into amplitude-modulation components of opposite polarities, and an arrangement for diierentially combining the outof the range substantially parabolic characteristics which similarly follow a substantialLv linear relation. Each of the frequencyresponsive networks has been designed to have a frequency-response characteristic of slope opposite to that of the other over the range of carrier frequencies applied thereto and to have substantially zero response at one frequency, maximum response at another frequency, and sub-A stantially linear response between these two frequencies. The use of linear rectiers with networks, of t nature has required that the points of zero and maximum response both lie outside of frequencies of the modulated carrier, since the combined output of the detectors is undistorted only so long as this istrue. 'Ihis not only has reduced the permissible range of carrier-signal frequencies but also has reduced the slope of the characteristic curve of the detector, thereby reducing its sensitivity.-

It is an object of the invention, therefore, to provide an improved frequency detector system which, while of general application, is especially suitable for use in a frequency-modulated carrier-signal receiver and one which avoids one or more of the above-mentioned disadvantages may include the frequencies of minimum response of the frequency-selective networks of the system.

It 1s an additionaiobjeet of the mvention'to provide an improved frequency detector system which includesa balanced pair of output circults, the output of each of which varies in a relation with the frequency of acarrier signal applied to the system,

puts of the two rectiflers to derive the frequency-k modulation components free fluctuation disturbances.

Certain prior art balanced frequency detectors of this nature have employed a pair of signalof any amplitude translating channels, each including an indi-i vidual frequency-selective network, the frequency-transfer characteristic of which is linear over a predetermined frequency range, andindisignal frequency detector system com frequency-selective stage responsive to a predewhereby. the differential of the outputsvaries in substantially linear` relation with the frequency of the carrier signal. f

In accordance with the invention, a carrierses a termined range of frequency deviation of a carrier signal applied thereto andincluding a pair of impedance networks having 'predetermined frequency-transfer characteristics of the same type but sloping in opposite senses .with frequency over a smaller frequency rangelying within and' symmetrically positioned with respect to the mean frequency of such predetermined range. The networks individually have minimum responses at opposite edges of the aforesaid smaller frequency range. There is also provided arectier stage including a pair of rectiiiers individually coupled to the impedance networks, the rectiers having similar predetermined inputoutput transfer characteristics. The transfer characteristics of either the frequency-selective stage or the rectifier stage follows substantially a linear relation and that of the other stage f'ollows a substantially square-law relation. The detector system has an output circuit including means for differentially combining the outputs of the rectifiers to derive an output having substantially linear variation with frequency over a range of frequency deviation including said predetermined range.

For a better understanding of the present invention, together with other and further objects thereof, reference is had to the following description, taken in connection with the laccompanying drawing, and its scope will be pointed out in the appended claims.

Referring now to the drawing, Fig. 1 is a circuit diagram, partly schematic, of a complete frequency-modulated carrier-signal receiver embodying the invention; Fig. 2 comprises a number of graphs representing the frequency-response characteristics of the frequency-selective frequency carrier signal in the oscillator-modulator I3, amplified in the intermediate-frequency amplifiers I4 and I5, and detected by the frequency detector system I6, I1, thereby to derive the audio-frequency modulation components. The audio-frequency components are, in turn, amplified in the audio-frequency amplifier I8 and are reproduced by the sound reproducer I8 in a conventional manner.

The intermediate-frequency carrier signalsA also are applied to the automatic amplification control or A. V. C. rectifier 28 to derive an automatic control or A. V. C. bias. 'Ihis bias is effective to control the amplification of one or more of the units I0, I3, and I4 to maintain the -signal input to the frequency detector I8, I1

stage, the rectifier stage, and the differential output of the detector system of the Fig. 1 arrangement; and Fig. 3 is a partial circuit diagram, partly schematic, of a modification of the frequency detector system of the invention.

Referring now more particularly to Fig. l, there is represented schematically a complete frequency-modulation carrier-signal receiver of a conventional design embodying the present in- .vention in a preferred form. In general, the receiver includes a. radio-frequency amplifier I8 having its input circuit connected to an antenna system II, I2 and having. its output circuit conv nected to an oscillator-modulator I3.- Connected in cascade with the oscillator-modulator I3,`in the order named, are an intermediate-frequency amplifier I4 of one or more stages, an additional stage of intermediate-frequency amplification I5, a frequency detector system I6, I1,- more fully described hereinafter, an audio-frequency amplifier I8 of one or more stages, and asound reproducer I9. An automatic amplification control or A. V. C. rectifier 28 also is connected to the intermediate-frequency amplier I4 and the outputof the rectiner is applied in conventional manner over the conductor indicated as A. V. C.

' to the input circuits of one or more of the tubes within a relatively narrow range for a wide range of received signalintensities.

Referring now more particularly to the portion of the system embodying the present invention, the output of the intermediate-frequency amplifier I4 is applied to an additional stage of intermediate-frequency amplification I5 which, if desired, may be so biased asV to operate with limitl ing action to limit to a predetermined amplitude level .carrier signals translated by this amplifier stage. vacuum tubes 2|, 22, the output circuits of which include load impedances 23, 24, respectively. The carrier signals developed across these load impedances are of opposing phase but of equal intensity and are, therefore, balanced voltages with respect to ground. These signals are applied through coupling condensers 25, 28 to a fre-l quency-selective stage I5 responsive over a predetermined range of frequency deviation of the carrier signals applied thereto and including a pair of impedance networks 21, 28. The impedance network 21 includes an inductor 28 shunttunedby a condenser 38 and a series-resonant shunt trap circuit comprising an inductor 3l and a condenser 32. Similarly, the impedance network 28 has an inductor 33 shunt-tuned by a condenser 34 and a series-tuned shunt trap circuit comprising an inductor 35 and a condenser 3S. The manner of tuning the resonant circuits of the'radio-frequency ampller I0, the oscillator-modulator I3, and the intermediate-frequency ampliiier I4.

It will be understood that the various units just described may, with the exception of the frequency detector system I6, I1, be of a conventional construction and operation, the details of which are well known in the art, rendering detailed description thereof unnecessary. Considering briefly the operation of the receiver as a.

whole, and neglecting for the moment the detailed.

operation of the frequency detector system I6, I1 presently tobe described, a desired frequencymodulated carrier signal is selectedl and amplified by the radio-frequency amplifier I0, converted to a frequency-modulated intermediateof the networks 21 and 28 will be described in greater detail hereinafter.

The output of the frequency-selective stage I8 is applied to a rectifier stage I1 which includes a pair of pentodevacuum tubes 31, 38 biased near cuto to operate as anode-circuit rectifiers for detecting amplitude variations. The output circuits of detectors 31, 38 include respective halves 39a, 39h of the primary winding of an audio-frequency transformer 40. Condensers 4I and 42 are provided to by-pass carrier-frequency currents around the respective halves 39a, 39h of the transformer primary winding. The transformer 48 is provided with a secondary winding 43.having a radio-frequency by-pass condenser 44 connected in parallel therewith, the secondary winding being connected in the input circuit 'of the audio-frequency amplifier I8 to apply thereto the modulation components' of the carrier signal.

The frequency-selective-or transfer characteristics of the impedance networks 21 and 28 are graphically represented in Fig. 2a. The network 21, which has a frequency-response characteristic as represented by curve d, is proportioned to have a major peak slightly above the upper edge of the range of frequency deviation f', f' of the intermediate-frequency carrier sig- The amplifier I5 comprises a pair of "magnitude but opposite phase.

These carriersignal potentials are applied to the frequencyselective networks 21 and 28 which superimpos'e on the frequencydeviations of the carrier signal the proportional amplitude variations in accordance with theirfrequency-selective characterisof frequency deviationand 4a minor peak above' the'upper edge. The series-tuned circuit 35, 35 is resonant at a frequencyV f2 within such range but above the mid-frequency fo thereof, the frequencies f1 and f2 being equidistant from the` mid-frequency fo. The resistors 23, 24 are effective to improve the linearity of each characteristic d, e within therange of frequency deviationA f f" by slightly rounding the' peaks. These characteristics have opposite slopes over the midfrequency range fifz or central portion of the ldevlation range, but their slopes are in the same sense over the end portions fi-f' and f-f" 'of the range, the slopes in the latter case beingboth equal and in the same sense. Hence, the frequency-transfer characteristics of the impedance networks 21 and 28 may be said to be ofthe same type but sloping in opposite' senses `with frequency over a smaller frequency range lying within and symmetrically positioned with respect tics, Fig. 2a. vThe outputs of the networks 21, 28 are applied to the respective rectifiers 31, 38,

which detect the resulting amplitude modulations of the carrier signals, the rectification occurring in each tube in accordance with its square-law transfer characteristic to provide detector outputs as represented fin'Fig. 2b. Curves g and h are parabolic overthe range. of linearity of curves e and d. The outputs of the detectors are differentially combined by the transformer to'derive a combined output which varies substanto the mean frequency of the range of frequency deviation of the carrier signalsapplied thereto, the networks individually having minimum responses at opposite edges of such smaller frequency range.

It may be noted at this point that the curves of Fig. 2a represent only the amplitudesof thel outputs of networks 21and 28 with frequency and neglectrthe polarity of the output. When the polarity of the output is additionally considered, it is well known in the art that the output of the` network 21, for example, has positive and negative polarities over individual portions A f-f1 and fr-f" of the frequency-deviation range f'-f",' for example, the output may bevconsidered to have positive polarity over the portion fi-f" and negative polarity over the portion f'-f1. Thus; if the curvesof Fig. 2a also took into account the polarity of the output, curve d would have the same slope over the frequency curve c should have the same slope over the frequency range f-f" and would` be a straight line throughout this range. Consequently, the inputoutput characteristics of the networks 21 and 28 are said to followa substantially linear relation with frequency over the range of frequency deviation of the carrier signal applied thereto.

The transfer characteristic of the detector system at the output of the rectiers 31 and38 of the rectifier stage I1 is represented in Fig.,'2b by curves y and h, respectively, these characteristics being curved and varying substantially in accordance with a square-law relation over the range of signal frequencies applied to the detector system from the networks 21, 28. The

vrange f-f" and the portion ofthe curve over this range would beY a straight line. Similarly.'

characteristics are similar and have equal and frequency amplier I5 and there is developed across the load resistors 23,24 in the output circuit thereof carrier-signal potentials of equal 75 tially linearly over the range 'of frequency devia.- tion of the carriersignal, asrepresented by the curve k of Fig. 2c,.the curvature of the two parabolic-shaped characteristics cancelling out in the combined output of the detector system. Itcan be shown mathematically that the difference of two parabolas having .their axis of ordinates parallel to, but 'displaced from, one another is a linear function. Thus, the curve 1c of. Fig. 2c is linear over the entire range of linearity of the curves d ande of Fig. 2a and its linearity is quite independent of the separation of the points of minimum response f1 and ,fzv of. the latter curves which determine only the displacement of the axes of ordinates of the parabolic curves y and h of Fig.v 2b and thereby the slope of curve Ic. The differentially combined outputs of the frequency-selective stage I6 and the rectifier stage l1 thus faithfully reproduce the wave form'of the frequency-modulation components. During intervals when the carrier signal is unmodulated, the frequency vof the carrier is at the mid-frequency for of the range of frequency deviati0n..so" amplitude nuctuatiOnS, such as might be caused' The modulationjcomponents appearing in the secondary winding 43 of the transformer 40 are applied to the input circuit of the audio-frequency amplifier I8 and, yafter amplification therein, are translated by the sound reproducer I9.

An amplitude-limiting system may be provided, if desired, at a point between the intermediatefrequency amplifier I4 and the amplier |5.` In this event, the amplifier. i5is biased to operate in normal manner without limiting action.

'I'he modification of the invention represented in Fig. 3 vdiffers from that Vof Fig. 1 in that the frequency-selective stagev 16a includes two frequency-responsive units 52, 54, each of which includes impedance networks similar' to those. of

unit I6 of Fig.V 1, whereby each of the units 52,. 54

has frequency-transfer characteristics `following 54 are connected in tandem by an intervening lunit 53 comprisinga coupling amplier similar to the amplifier I5. One impedance network of unit 52 and a corresponding impedance network of unit 54 together comprise one of a pair of impedance networks of the frequency-selective stage Isa, each such pair of networks having an over-all transfer characteristic following Asubstantially a square-law relation, not graphically shown but similar to that of the detector transfer characteristics graphically represented by the curves of Fig. 2b. The amplifier lil may be biased 4 I f sacaste modifications may be made therein without deto operate with limiting action, if desired, but

the amplifier of unit 53 must be biased to operate on a linear .portion of its characteristic curve since otherwise it would destroy the selective characteristics of the preceding frequency-responsive unit 52. vConnected to the output of the frequency-selective stage lia is a rectifier stage 45 comprising a pair of diode rectifiers 46, 41,con-

nected with opposing polarity, and respective.

diode load resistors 48, 49. The outputs of the rectifiers 46, 4l are averaged by a pair of seriallyconnected resistors 50, 5I connected across the load resistors 48, 49 and the demodulation components of the carrier signal appear between the junction of the resistors 5l),` 5| and the junction ofthe resistors 48, 49, the output being the average of the unidirectional potentials developed with opposing polarities across the diode load n resistors48, 49. The transfer characteristic of the rectifier stage 45 follows substantially a linear relation over the range of frequencies of the carrier signal applied to the detector system. The transfer characteristic of the detector system at the output of each rectifier 46, 41 is thus as represented by curves aand h of Fig. 2b, and the output of the frequency detector system,

comprising the frequency-selective stage lia,v

and the rectifier stage 45, is linear over the range of frequency deviation of the carrier signal, as represented by the curve'k of Fig. 2c, whereby parting from the invention, and it is, therefore, aimed in the appended claims to cover all such changes and modifications as fall within the trueA spirit andscope of the invention.

What is claimed is:

l. A carrier-signal frequency detector system comprising,4a frequency-selective vstage responsiveover a predetermined range of frequency deviation of a carrier signal applied thereto and including a pair of impedance networks having predetermined frequency-transfer characteristics ofthe same type but sloping in opposite senses with frequency over a smaller frequency range lying -within and symmetrically positioned with mined range, said networks individually having minimum responses at opposite edges of said smaller frequency range, a rectifier stage including a pair of rectiiiers individually coupled to said impedance networks, said rectiiers having similar predetermined input-output transfer characteristics, the transfer characteristic of one the wave form of the modulation components of the carrier signal is faithfully reproduced.

It will be evident from the foregoing description of the invention that a carrier-signal necelver embodying the invention is capable of satisfactory operation over avery wide range of frequency modulation, such as the'range f'f" of Fig. 2a, and is, therefore, not limited to the comparatively narrow frequency range of the prior frequency detectors which lies between the frequencies of minimum `response fr. haof the frequency-selective networks of the detector system.l The output of the frequency detector 'system of the invention is proportional-to the sepavration of the frequencies l1, f2 of minimum response, so that it is not desirable to have these 'points of minimum response too close together.

On the other hand, the. rectied output of the' Vdetector system inthe center of the band of frequency deviation, which output has to be cancelled in the differential output circuit of the detector, is proportional to the square of the frequency separation of the points of minimum frequency response of fr, f2, -so that it becomes desirable to have these points not too greatly separated. As a compromise, the points of minimum frequency response fi, fz should be separated by an amount approximately equal to the normalrange of frequency deviation of the carriersignal.

This separation may, however, be slightly greater or slmhtly less without introducing distortion. In any event, the separation of the minimum re l sponse fi, fz is not critical insofar as the linearity of the detector output characteristic is concerned, the linearity being simply the result of differentially combining detector output characteristics which have similar parabolic form over the linear portions of the individual impedance network frequency-transfer characteristics. a

While there have been described .what are at present considered to be the preferred embodiments of this invention,l it will be obvious to those skilled in the art that various changes and of said stages following a substantially squarelaw relation and that of the other of said stages following a. substantially linear relation, and an output circuit including means for differentially combining the outputs of said rectiflers to derive an output having substantially linear variation with frequency over a range of frequency deviation including said predetermined range.

2. A carrier-signal frequency detector system comprising, a frequency-selective stage including a pair of impedance networks having predeterstantially a square-lawrelation and that of the other of said stages following substantially a linear relation, and anoutput circuit including means for differentially combining the outputs of said rectifiers to derive an output having substantially linear variation with frequency over a range of frequency deviation including said range.

3. A carrier-signal frequency detector system I comprising, a frequency-selective stage responsive over a predetermined range of frequency deviation of a carrier signal appliedthereto and including a pair of impedance networks having similar frequency-transfer characteristics substantially linear but sloping in opposite senses with frequency over a smaller frequency range lying within-and symmetrically positioned with respect -to the mean frequency of said predetermined range,I said networks individually having minimum responses Vat opposite edges ofl said smaller frequency range, a rectifier stage including a pair of rectifiers individually coupled to said impedance networks, said rectiflers having similar substantially square-law input-output transfer characteristics, and an output circuit including means for differentially combining the out-4 puts ofsaid rectifiers to derive an output having 4. A carrier-signal frequency deteetnr'system frequency response intermediate said rangei the comprising, a frequency-selective stage responpoint of minimum response being displaced from sive over a predetermined range of frequency dethe mean frequency of said range on the side'- viation of a carrier signal applied theretp and thereof opposite the major response peak of the including a pair of impedance networkshaving 5 individual network, a rectifier stage including a similar substantially square-law frequency-ftranspair of rectifier-s individually coupled to said imfer characteristics but sloping in opposite senses l pedance networks, said rectifiers having similar with frequency over a smaller lfrequency range Substantial? seguirle-lull?l input-Output lying within and symmetrically positioned with characteristics; and an output circuit including respect to the mean frequency Vof said predeterl04 means for differentially combining the outputs mined range, -a rectiiier stage including a pair v of said rectifiers to derive an output having subof/rectiers individually coupled to said imstantially linear variation with frequency over a pedance networks, said rectiiiers having similar range of frequency deviation including said range.

substantially linear input-output transfer char- 7,:A carrier-signalv frequency detecter Systemacteristics, and an output circuit including vmeans l5 comprising, a frequency-selective stage responfor differentiallyv combining the outputsof Vsaid rectiilers -to derive an output having substantially linear variation with frequency over a range-of sive-over'aipredetermined range of frequency de- .ietie including xanga. I y 20 ing substantially a linear relation but sloping in 5. 'A carrier-signal frequency detector*sililjteni'hg` comprising, 'a frequency-selective stage including#` queney yrenee lying within andv symmetrically barrier signal applied thereto undv Y pair of 'impedance networks having `frequency deviation including said predetermined similar frequency-transfer characteristics followopposite s"enses with frequency overa smaller fre" a pair of impedance networks -eachiindivicluadly).'l: positioned with respect to the mean frequency of tuned to have maximum responses at opposite said predetermined range, said lnetworks 'indiends of a predetermined range'of frequency 1de' 25 .vidually having minimum responses at opposite viation of a carrier sig'nalapplied thereto and L :edges-of said smaller frequencyqrange, a rectier said impedance networks each having a minimum a Stage including n Pull' 0f Vacuum tubes individualresponse at respective opposite ends ofl aninterly ycoupled to said impedance networks. said mediate smaller range of frequencies symmetri-Lg vacuum tubes .being biased near cutoif to provide cally positioned about the mid-frequency of .said 30 similar. Asubstantiel1l? Square-law 'input-Output 4' range, whereby .the frequency-transfer cnarac- .transfer characteristics, and an output circuit teristics of said networks slope in opposite senses ineludin8 means for differentially combining the over said intermediate range of frequencies,v a.: outputscf said rectiilers to derive4 an output rectifier stage including a pair of rectiflers in- Varying Substantially linearly withfrequency over dividually `coupled'tc said impedance networks35 a range of frequency d eviatisnincluding said- .said rectiers having similar predetermined inpredetermined range.

.putentput transfer characteristics, then-ansteii 8.141- ea'rrier-sisn'al frequencvfdetector system .characteristic of one of said stages following sub comprising, ,e frequenti-Selective stese including stantiaily a square-law relation and that of the. e pair efgfimpedeneenetworks hevins similar pref other of said stages'l following substantially a l0-determined frequency-transfer characteristics linear relation, and an output circuit including xi'ellewins.. Substantially .a square-lawy relation means for differentially combining the outputs over u flJredeterlnined renge'offrequency devia' or said .restiners to derive an output having sub-4 .tion-offs .earrier'sisnaI applied thereto, a rectier stantially linearl variation with frequency over a? y stage including .e Pair 0f diode. reetlers indii range of frequency deviation including said determined range.

fvidually coupled 'to said impedance networks',

said. diode rectifiers having similar substantially` 6. A carrier-signal frequency detectar System linear input-output transfer characteristics, and

comprisingga frequency-selective stage including an output circuit including means. for differena, pair 0f impedance networks each having a l1219.115' combining the Outputs 0f s aid rectiels t0 shunt-tuned circuit and a series-timed shunt 50 derive en Output varying substantially linearly .trap circuit so proportioned that each network "With frequency Over e renee 0f frequency de via- I has a major frequency-response peak on oneedge tion including said range.

and a minor response peak on the opposite edge of a range of frequency deviation of a carrier A `HAROLD A..

vsignal applied thereto and a point of minimum 55 

